Apparatus and method of controlling motor in manner resistant to additive noise during start up of motor

ABSTRACT

An apparatus for controlling a rotational motion of a motor is disclosed which includes: a signal generator generating a rotational state signal indicative of a rotational state of the motor; and a controller producing a control signal for controlling the rotational motion of the motor, based on the rotational state signal generated by the signal generator, to thereby control the motor based on the produced control signal. The controller produces the control signal during a start up of the motor, such that the produced control signal is not affected by a noise component which is incorporated into the rotational state signal during the start up of the motor.

This application is based on Japanese Patent Application No. 2004-098570filed Mar. 30, 2004, the content of which is incorporated hereinto byreference.

CROSS-REFERENCE TO RATED APPLICATIONS

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a technique of controlling a rotational motionof a motor based on a signal indicative of the rotational state of themotor, and more particularly to a technique of controlling the motor ina manner resistant to an additive noise.

2. Description of the Related Art

Typically, various kinds of machines and apparatuses using motors needcontrol of the respective motors. For fulfilling such a need, atechnique has been widely practiced in which a rotational state signalis generated indicative of the rotational state of the motor, and inwhich the rotational motion of the motor is controlled based on thegenerated rotational state signal.

An example of such a machine is an image forming apparatus of anelectrophotographic type such as a laser printer. The image formingapparatus is configured to include a polygon mirror for use indeflecting a laser beam exiting a semiconductor laser, a photosensitivedrum on which an electrostatic latent image is formed with the laserbeam exiting the polygon mirror, etc. These elements are eachcategorized into a movable element. Such a movable element requires aprecise control of the rotational speed of the movable member. The imageforming apparatus may include other elements each having the samerequirement.

One example of a technique of controlling the rotational state of amotor has been commonly carried out, in which a frequency generator isemployed to output a frequency signal indicative of the rotational speedof the motor, and in which the rotational motion of the motor iscontrolled through feedback control, based on the frequency signaloutputted from the frequency generator.

BRIEF SUMMARY OF THE INVENTION

However, the possibility exists that the frequency signal fails toreflect the actual rotational state of the motor accurately during astart up of the motor. One of the reasons is that unexpected reverserotations of the motor occur due to such as backlash in a motiontransmission system of the motor.

A technique for use in an image forming apparatus is disclosed as oneexample of a conventional technique available in an environment wherethe above possibility exists, in which open-loop control is performedduring an initial period of the operation of the motor, and in which theopen-loop control is switched into closed-loop control after apredetermined time elapses from the preceding open-loop control.

However, an image forming apparatus, once being activated using theabove conventional technique, is operated such that, during a start upof the motor, the open-loop control is performed for the motor byoutputting thereto a control signal which has been produced as a resultof a complete ignorance of information relating to the actual rotationalstate of the motor.

That is, the above conventional technique is established, in light ofthe possibility that the actual rotational state of the motor during itsstart up contains an additive noise, to prevent effects of the additivenoise on the controlled rotational state of the motor during its startup.

When the above conventional technique is practiced, there is nomonitoring the actual operating state of the motor during the open-loopcontrol, resulting in incapability of detecting a possible abnormalityin the motor operation during its start up due to some reason. Ingeneral, an abnormality in the motor operation during its start up mayinvite a drawback during the subsequent closed-loop control.

Despite of that, the above conventional technique performs theclosed-loop control irrespective of whether or not the preceding startup of the motor was normally experienced. For this reason, anabnormality in the rotational state of the motor during its start up maypossibly prevent an expected entry of the motor control into theclosed-loop control.

Further, the above conventional technique requires a shifting of thecontrol manner of the motor between the open-loop and the closed-loopcontrol over a continuous period of operation of the motor, resulting inincrease in structural complexity and manufacturing cost.

It is therefore an object of the present invention to provide atechnique of controlling the rotational state of a motor based on arotational state signal indicative of the rotational state of the motor,in an expected condition, despite of the generation of a noise in therotational state signal during a start up of the motor.

According to the present invention, an apparatus for controlling arotational motion of a motor is provided, comprising:

-   -   a signal generator generating a rotational state signal        indicative of a rotational state of the motor; and    -   a controller producing a control signal for controlling the        rotational motion of the motor, based on the rotational state        signal generated by the signal generator, to thereby control the        motor based on the produced control signal,    -   wherein the controller produces the control signal during a        start up of the motor, such that the produced control signal is        not affected by a noise component which is incorporated into the        rotational state signal during the start up of the motor.

It is possible to predict to a certain degree of accuracy thecharacteristics (e.g., frequency) of a noise which is incorporated intoa rotational state signal indicative of the rotational state of themotor during its start up. In view of this, a noise component generatedduring the start up of the motor can be reduced in level in therotational state signal of the motor.

Based on the above findings, the apparatus according to the presentinvention allows the production of the control signal so as to avoid anadverse effect on the resulted control signal, of the noise componentwhich has been introduced into the rotational state signal during thestart up of the motor.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments of the invention, will be better understood whenread in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities show. In the drawings:

FIG. 1 is a perspective and exploded view illustrating a laser printeras an image forming apparatus according to a first embodiment of thepresent invention;

FIG. 2 is a sectional side view illustrating the laser printer shown inFIG. 1;

FIG. 3 is a perspective view illustrating a drive train unit shown inFIG. 1;

FIG. 4 is a schematic view illustrating a motor control device in thelaser printer shown in FIG. 1;

FIG. 5 illustrates in wave form charts output signals of a frequencygenerator, an amplifier, an input buffer, and a noise reducer, all ofwhich are shown in FIG. 4, for explanation of a noise reductionperformed by the noise reducer;

FIG. 6 illustrates in wave form charts an input signal of the noisereducer shown in FIG. 4, and two output signals of the noise reducerwhich are for explaining variations in output signal of the noisereducer depending on the magnitude of a set value of the noise reducer;

FIG. 7 is a perspective view illustrating a laser optical system of alaser printer according to a second embodiment of the present invention:

FIG. 8 is a schematic view illustrating a motor control device in thelaser printer shown in FIG. 7: and

FIG. 9 illustrates in wave form charts output signals of a beamdetector, a detector-signal holder, and a noise reducer, all of whichare shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The object mentioned above may be achieved according to any one of thefollowing modes of this invention.

These modes will be stated below such that these modes are sectioned andnumbered, and such that these modes depend upon the other mode or modes,where appropriate. This is for a better understanding of some of aplurality of technological features and a plurality of combinationsthereof disclosed in this description, and does not mean that the scopeof these features and combinations is interpreted to be limited to thescope of the following modes of this invention.

That is to say, it should be interpreted that it is allowable to selectthe technological features which are stated in this description butwhich are not stated in the following modes, as the technologicalfeatures of this invention.

Furthermore, stating each one of the selected modes of the invention insuch a dependent form as to depend from the other mode or modes does notexclude a possibility of the technological features in a dependent-formmode to become independent of those in the corresponding depended modeor modes and to be removed therefrom. It should be interpreted that thetechnological features in a dependent-form mode is allowed to becomeindependent according to the nature of the corresponding technologicalfeatures, where appropriate.

(1) An apparatus for controlling a rotational motion of a motor,comprising:

-   -   a signal generator generating a rotational state signal        indicative of a rotational state of the motor; and    -   a controller producing a control signal for controlling the        rotational motion of the motor, based on the rotational state        signal generated by the signal generator, to thereby control the        motor based on the produced control signal,    -   wherein the controller produces the control signal during a        start up of the motor, such that the produced control signal is        not affected by a noise component which is incorporated into the        rotational state signal during the start up of the motor.

As described above, it is possible to predict to a certain degree ofaccuracy the characteristics (e.g., frequency) of a noise which isincorporated into a rotational state signal indicative of the rotationalstate of the motor during its start up. In view of this, a noisecomponent generated during the start up of the motor can be reduced inlevel in the rotational state signal of the motor.

Based on the above findings, the apparatus according to the above mode(1) allows the production of the control signal so as to avoid anadverse effect on the resulted control signal, of the noise componentwhich has been introduced into the rotational state signal during thestart up of the motor.

(2) The apparatus according to mode (1), wherein the controller producesthe control signal for controlling the rotational motion of the motorthrough feedback control based on the generated rotational state signal,irrespective of whether or not the start up of the motor is beingexperienced.

The apparatus according to the above mode (2) allows the feedbackcontrol of the rotational motion of the motor using the rotational statesignal irrespective of whether or not the start up of the motor is beingexperienced.

The apparatus according to the above mode (2) therefore does not requirea shifting of the control manner of the motor from open-loop control toclosed-loop control, resulting in avoidance of an inappropriateclosed-loop control after and due to an inadequate start up operation ofthe motor resulting from the precedent open-loop control.

(3) The apparatus according to mode (1) or (2), wherein the controllercomprises: a noise reducer reducing a level of the noise component inthe rotational state signal generated by the signal generator; and acontrol-signal producing device producing the control signal based onthe rotational state signal in which the noise component has beenreduced by the noise reducer.

The apparatus according to the above mode (3) allows the control of themotor during its start up by a noise reduction of a noise componentwhich is predicted to be generated during the start up of the motor,wherein the noise reduction is performed in the rotational state signal,i.e., information relating to the rotational state of the motor.

The apparatus according to the above mode (3) therefore allows a moreappropriate control of the rotational state of the motor using thedetected rotational state signal of the motor during its start up,despite that the detected rotational state signal is originally possibleto contain a noise component during the start up of the motor.

As a result, the apparatus according to the above mode (3) permits theprevention of entry of the motor control into closed-loop control, i.e.,feedback control at the time that the rotational motion of the motor isnot yet adequately controlled.

(4) The apparatus according to mode (3), wherein the noise reducerreduces the level of the noise component in the rotational state signal,prior to the production of the control signal based on the rotationalstate signal.

(5) The apparatus according to mode (3) or (4), wherein the noisereducer is configured such that a noise component to be reduced in levelin the rotational state signal is varied in characteristics depending ona set value, and wherein the set value is varied to be different betweenduring and after the start up of the motor.

The apparatus according to the above mode (5) allows the temporalvariation in characteristics (e.g., frequency) of a noise componentwhich is to be reduced by the noise reducer, contributing to anoptimized noise reduction not only during the start up operation of themotor but also during the regular operation of the motor.

In the apparatus according to the above mode (5), the temporal variationin characteristics of the noise component is achieved by the temporalmodification of a set value of the noise reducer.

According to one of applications of the set value, the rotational statesignal is converted into a binary signal, and then whether the binarysignal is at high level or low level is determined. If the binary signalis at high level, the count value of a counter is incremented each cycleof a reference clock, while, if the binary signal is at low level, thecount value is decremented each cycle of the reference clock.

In the above application of the set value, until the count value reachesthe set value, the noise reducer holds the level of its output signaldespite of a change or transition in level of its input signal. If thecount value reaches the set value, the noise reducer allows a change ortransition in level of the output signal in response to a change ortransition in level of the input signal.

Although the above application of the set value employs the noisereducer in the form of the counter, the noise reducer in the above mode(5) may be modified to a customary digital filter such as a digitalfilter of an FIR type or an IIR type. In this modification, it may beconsidered that a filter coefficient or factor of the applied digitalfilter corresponds to one example of the set value set forth in theabove mode (5).

In practicing the apparatus according to the above mode (5), the setvalue may be defined so as to change at a time that a predeterminedlength of time is elapsed, a time that the rotational speed of the motorrepresented by the rotational state signal exceeds a predeterminedspeed, or the like.

(6) The apparatus according to mode (5), wherein the set value isestablished in magnitude to allow the noise reducer not to reduce inlevel a frequency component of the rotational state signal whichreflects a true component of compound rotational motion of the motorduring the start up of the motor.

The apparatus according to the above mode (6) allows the noise reducerto pass a true component of the compound rotational motion of the motor,wherein the true component reflects a normal operation of the motorduring its start up.

The rotational speed of the motor is lower during the start up operationthan that during the regular operation of the motor following the startup operation. As a result, in the case where the rotational state signalis in the form of the aforementioned frequency signal, the frequency ofthe true component of the rotational state signal is lower during thestart up operation than that during the regular operation.

With this in mind, the apparatus according to the above mode (6) may bepracticed such that, during the start up of the motor, the noise reducerreduces a high frequency component (i.e., noise component) of therotational state signal, while passing a low frequency component (i.e.,true component) of the rotational state signal, allowing an appropriatedetection of the rotational state of the motor during its start up.

(7) The apparatus according to any one of modes (3)-(6), wherein thenoise reducer comprises an amplifier amplifying the rotational statesignal, wherein the amplifier is configured to become substantiallyinoperative during the start up of the motor.

The apparatus according to the above mode (7) allows the noise reductionusing an amplifier variable in operation state. In practicing theapparatus, the noise reducer may be constructed to additionally includea filter performing noise reduction depending on the set value asdescribed above.

(8) The apparatus according to any one of modes (3)-(6), wherein thenoise reducer comprises an amplifier amplifying the rotational statesignal by a variable amplification factor, wherein the amplificationfactor is established to become smaller during the start up of themotor, and become larger during a steady motion of the motor.

The apparatus according to the above mode (8) allows the noise reductionusing an amplifier variable in amplification factor. The amplificationfactor is varied so as to be smaller during the start up of the motor,and larger during a steady or regular motion of the motor.

According to an example of the apparatus according to the above mode(8), the amplification factor may be varied depending on the amplitudeof the rotational state signal, such that the amplification factorbecomes smaller with the amplitude being smaller, and larger with theamplitude being larger. Such an amplifier employed in the example may bean antilogarithmic amplifier.

The apparatus according to the above mode (8) may be practiced in anarrangement where a three-phase brushless motor may be employed as themotor, a frequency generator may be employed as the signal generator,and additionally, a frequency signal of the frequency generator operableusing magnetic patterns (FG patterns) on the outer circumferentialperipheral of a rotor of the three-phase brushless motor may be employedas the rotational state signal.

In this arrangement, the amplitude of the frequency signal is largerwith the rotational speed of the motor being lower, while is smallerwith the rotational speed is higher. This arrangement provides apreferable situation where the above example of the apparatus accordingto the above mode (8) is available.

In consideration of the possibility that the rotational state signal isamplified prior to entry into the noise reducer, the term “rotationalstate signal” to be directly processed by the noise reducer set forth inany one of modes (2) to (7) may be interpreted to mean the same signalas the raw rotational-state-signal (e.g., a frequency signal) generatedby the signal generator, or to mean the signal obtained by modifying oramplifying the raw rotational-state-signal generated by the signalgenerator.

(9) The apparatus according to mode (8), wherein the amplifier is anantilogarithmic amplifier.

(10) The apparatus according to any one of modes (1)-(9), wherein thesignal generator comprises a frequency generator outputs the rotationalstate signal in the form of a frequency signal which is an analog signalvaried in frequency depending upon the rotational state of the motor.

(11) The apparatus according to mode (10), wherein the frequency signalis varied in frequency depending upon a rotational speed of the motor.

(12) The apparatus according to mode (10) or (11), wherein thecontroller comprises a noise reducing device reducing in level the noisecomponent in a signal obtained by amplifying the frequency signalgenerated by the frequency generator.

(13) The apparatus according to any one of modes (1)-(12), wherein themotor comprises a scanner motor rotating a polygon mirror for use inscanning a light beam cyclically, wherein the signal generator comprisesa beam detector disposed stationary at a predetermined position allowingthe light beam enters the beam detector intermittently per each cycle ofscanning of the light beam, the beam detector outputting a detectorsignal varied in level depending on whether or not the beam detectorreceives the light beam, and wherein the rotational state signal is thedetector signal outputted from the beam detector.

The apparatus according to the above mode (13) may be practiced invarious manners. For example, the apparatus may be practiced in a mannerthat the scanner motor deflects a laser beam emitted from asemiconductor laser as a light source. The apparatus may be practicesuch that the controller performs the noise reduction of the same as theraw detector-signal generated by the beam detector, or of the signalobtained by modifying or amplifying the raw detector-signal generated bythe beam detector. The apparatus may be practiced such that the beamdetector is for use in control of scanning operation for exposure of aphotoconductor (e.g., a photosensitive drum, a photosensitive belt,etc.).

(14) The apparatus according to mode (13), wherein the polygon mirrordeflects the light beam cyclically to scan a photoconductor.

(15) The apparatus according to mode (13) or (14), wherein thecontroller comprises:

-   -   a signal processor processing the detector signal outputted from        the beam detector:    -   a noise reducing device reducing the noise component in the        detector signal processed by the signal processor; and    -   a control-signal producing device producing the control signal        based on the detector signal in which the noise component has        been reduced by the noise reducing device,    -   wherein the signal processor processes unprocessed detector        signal into processed detector signal in a manner that, in a        steady state of the unprocessed detector signal, the processed        detector signal indicates whether the unprocessed detector        signal is a non-light-reception signal indicating that the beam        detector does not receive the light beam, or a light-reception        signal indicating that the beam detector receives the light        beam,    -   and wherein the signal processor processes the unprocessed        detector signal into the processed detector signal in a manner        that, in an oscillating state in which the unprocessed detector        signal is oscillated in level so as to alternately repeat a        forward transition of the unprocessed detector signal from the        non-light-reception signal to the light-reception signal, and a        reverse transition from the light-reception signal to the        non-light-reception signal, the processed detector signal        steadily indicates the forward transition, without indicating        the reverse transition, during a predetermined length of time        elapsed from a start time of the forward transition.

In the apparatus according to the above mode (15), the beam detectoroutputs an on-state signal as an example of a light-reception signalindicating that the beam detector received the light beam. The lightbeam does not resident at the beam detector, but momentarily passesthrough the beam detector, resulting in the beam detector outputting theon-state signal only for a very short time.

In view of the above, the apparatus according to the above mode (15)performs a particular signal processing for the output signal of thebeam detector, for decreasing the sensitivity of the input signal of thenoise reducing device to the output signal of the beam detector. This isconducive to a more ensured operation of the noise reducing device.

(16) An apparatus for forming an image, comprising;

-   -   an image forming device forming an image on an image receiver        medium;    -   a feeding device feeding the image receiver medium to the image        forming device;    -   at least one motor for use in at least one of the image forming        device and the feeding device; and    -   a driving device driving the at least one motor for rotation        thereof,    -   wherein the driving device comprises the apparatus according to        any one of modes (1)-(15) for use in controlling at least one of        the at least one motor.

The apparatus according to the above mode (16) improves the controlledstate of at least one motor, eventually resulting in an improvedperformance in forming an image, for example.

(17) A method of controlling a rotational motion of a motor, comprisingthe steps of:

-   -   generating a rotational state signal indicative of a rotational        state of the motor;    -   reducing within the generated rotational state signal a level of        a noise component which is incorporated into the rotational        state signal during a start up of the motor;    -   producing a control signal for controlling the rotational motion        of the motor through feedback control, based on the rotational        state signal in which the noise component has been reduced in        level, irrespective of whether or not the start up of the motor        is being experienced; and    -   controlling the motor based on the produced control signal.

The method according to the above mode (17) provides basically the samefunctions and effects as those of the apparatus according to the abovemode (1), according to basically the same principle as that of theapparatus.

Several presently preferred embodiments of the invention will bedescribed in more detail by reference to the drawings in which likenumerals are used to indicate like elements throughout.

FIG. 1 shows in perspective view a laser printer as an image formingapparatus including a motor control device in accordance with a firstembodiment of the present invention. FIG. 2 shows the laser printer insectional side view.

As shown in FIG. 1, the laser printer includes a body case 1 made ofsynthetic resin. The body case 1 has a main frame 1 a and a main cover 1b which covers externally the main frame 1 a on all sides. The mainframe 1 a and the main cover 1 b are integrally formed using a methodsuch as an injection molding.

The main frame 1 a includes: a front side portion (on the near side ofFIG. 1); a rear side portion (on the far side of FIG. 1); a left sideportion (on the right-hand side of FIG. 1); and a right side portion (onthe left-hand side of FIG. 1). The main frame 1 a further includes aninner space surrounded by the front, rear, left, and right sideportions.

As shown in FIG. 1, an operation panel 1 c is disposed at the main frame1 a. More specifically, the operation panel 1 c is disposed on the uppersurface of a projection extending upwardly from the upper surface of theleft side portion of the main frame 1 a.

As shown in FIG. 2, on the main frame 1 a of the body case 1, there aremounted a scanner unit 2 as an exposure device; a process unit 3 as animage forming device; a fuser unit 4; a feeder unit 5; a drive trainunit 6 (see FIG. 1); etc.

In the present embodiment, as shown in FIG. 3, the drive train unit 6 isconstructed so as to include a main motor 6 a and a gear train 6 b. Theconstructions equivalent or similar to those of the main motor 6 a andthe gear train 6 b are disclosed in U.S. Pat. No. 6,205,302, the contentof which is incorporated hereinto by reference. The main motor 6 a is inthe form of a three-phase brushless DC motor controlled by the motorcontrol device described later in more detail.

As shown in FIG. 1, a storage recess 1 d is formed between an innersurface of the right side portion of the main cover 1 b shown in FIG. 1and an outer surface of the right side portion of the main frame 1 a inproximity to each other. The storage recess 1 d accommodates the drivetrain unit 6 therein. The drive train unit 6 is inserted from under themain case 1 for attachment and fixing thereto.

As shown in FIG. 1, the laser printer further includes a top cover 7 asa body cover made of synthetic resin, which covers the upper surfaces ofthe main frame 1 a and the main cover 1 b. At the top cover 7, throughholes 7 a and 7 b are formed. The through hole 7 a is formed to allowthe aforementioned projection of the main frame 1 a to upwardlypenetrate the top cover 7, whereby the operation panel 1 c is exposed toover the upper surface of the top cover 7. On the other hand, thethrough hole 7 b is formed to allow a base portion of the feeder unit 5to penetrate the top cover 7.

As shown in FIGS. 1-2, the laser printer further includes an exit tray8. The exit tray 8 is mounted at its base end portion on a pair ofsupports 9, 9 (only one of which is shown in FIG. 1) so that the exittray 8 may be pivotable in a general vertical direction. The supports 9,9 are formed on a front portion of the top cover 7 at both lateral endsthereof, respectively. When not in use, the exit tray 8 can be foldedback onto and cover the upper surface of the top cover 7.

As shown in FIGS. 1-2, the feeder unit 5 has a feeder case 5 a, asupport plate 10, and a feeder roller 11. Into the feeder case 5 a, astack of individual recording sheets P as receiving media located oneagainst another is loaded. The feeder unit 5 further has a spring 10 awithin the feeder case 5 a which biases the support plate 10 toward thefeeder roller 11. As is evident from FIG. 2, a leading edge portion of arecording sheet P is pressed toward the feeder roller 11 via the supportplate 10. The feeder roller 11 is rotated because of a driving forcetransmitted from the drive train unit 6.

As shown in FIG. 2, the feeder unit 5 further includes a separation pad12, and a pair of registration rollers 13 and 14 opposing to each other.Because of coaction of the feeder roller 11 and the separation pad 12,the recording sheets P, upon separated from one another one by one, aredelivered to between the registration rollers 13 and 14.

The feeder unit 5 further includes a sheet inlet 5 b for a manual feedof a recording sheet P. The sheet inlet 5 b allows the user who wishesto print a selected recording sheet P different in type from therecording sheets P previously set within the feeder case 5 a, to insertthe selected recording sheet P into the feeder unit 5 via the sheetinlet 5 b.

The process unit 3, in operation, attaches a toner as a developermaterial to the surface of the recording sheet P which is fed into theprocess unit 3 by means of the registration rollers 13 and 14, resultingin formation of a toner image on the recording sheet P.

The fuser unit 4 includes a heat roller 15 and a pressure roller 16opposing to each other. The fuser unit 4, in operation, heats therecording sheet P on which the toner image has been formed, with therecording sheet P being in contact with the heat roller 15 and thepressure roller 16 therebetween, resulting in fixing of the toner imageonto the recording sheet P.

The heat roller 15 is constructed to include a fuser heater 15 a withinan aluminum pipe whose surface is coated with fluorine. A thermistor 25as a temperature sensor is disposed in contact with the outer surface ofthe heat roller 15 at an approximately axially center position thereof.The pressure roller 16 is in the form of a rubber roller whose surfaceis coated with fluororesin.

As shown in FIG. 2, the fuser unit 4 further includes within its case anexit roller 17 and a pinch roller 18 both of which are disposed on adownstream side of a travel path of the recording sheet P. The exitroller 17 and the pinch roller 18 cooperate to constitute a sheetejector of the fuser unit 4. The sheet ejector ejects the recordingsheet P onto which the toner image has been fused, to the exit tray 8 Inthe present embodiment, a path extending from the feeder roller 11 tothe sheet ejector is the travel path of the receiving media (illustratedin dash-dot-dot lines in FIG. 2).

As shown in FIGS. 1-2, the process unit 3 is disposed in the main frame1 a at an approximately center position thereof. As shown in FIG. 2, thescanner unit 2 is disposed under the process unit 3. The scanner unit 2is fixed at its upper support plate 2 a, using suitable fixtures such asscrews not shown, to a stay portion which is formed integrally with andon the upper surface of the bottom plate of the main frame 1 a.

As shown in FIG. 2, the scanner unit 2 as an exposure device is composedof a laser emitter not shown; a polygon mirror 20; a lens 21; reflectivemirrors 22, 22; etc., all of which are disposed within the scanner unit2 under its upper support plate 2 a made of synthetic resin. The polygonmirror 20 is driven for rotation at a higher speed by a scanner motor 86in the form of a three-phase brushless DC motor. The scanner motor 86 isdriven by a motor drive circuit 90 including the motor control devicedescribed later in more detail.

The polygon mirror 20 reflects a laser beam emitted from theaforementioned laser emitter, and also deflects the laser beam toangularly oscillate it. The deflected laser beam passes one of thereflective mirrors 22, 22, the lens 21, and the other of the reflectivemirrors 22, 22, sequentially in the description order.

As shown in FIG. 2, the laser beam then enters a glass plate 24 whichcovers an elongate slot formed through the upper support plate 2 a. Theslot is formed in the upper support plate 2 a so as to extend parallelto the axis of a photoconductive or photosensitive drum 23 as aphotoconductor of the process unit 3. The laser beam, upon passingthrough the glass plate 24, enters the surface of the photosensitivedrum 23. The surface of the photosensitive drum 23 is cyclically scannedwith the laser beam, resulting in exposure of the photosensitive drum 23to the laser beam.

As shown in FIG. 2, the process unit 3, as described above, includes thephotosensitive drum 23. In the process unit 3, a transfer roller 25 isdisposed in abutment against the upper surface of the photosensitivedrum 23. Under the photosensitive drum 23, a charger 26 (which is of aScorotron-type, for example) is additionally disposed.

The process unit 3 further includes: a developer having a developerroller 27 and a supply roller 28 both of which are disposed upstreamfrom the photosensitive drum 23 in the feeding direction of therecording sheet P; and a toner supply portion, that is to say, aremovable toner cartridge 29 disposed upstream from the developer. Theprocess unit 3 yet further includes: a cleaning roller 30 disposeddownstream from the photosensitive drum 23; a discharge lump 30 adisposed downstream from the cleaning roller 30; etc.

The uniform charge of the surface of the photosensitive drum 23 by thecharger 26 forms a photosensitive layer on the surface of thephotosensitive drum 23. The photosensitive layer is scanned with thelaser beam exiting from the scanner unit 2, resulting in formation of anelectrostatic latent image on the photosensitive layer. A toner withinthe toner cartridge 29, upon agitated with an agitator 31, exits fromthe toner cartridge 29, and then is carried on the surface of thedeveloper roller 27 via the supply roller 2B to form a toner layer. Thetoner layer carried on the surface of the developer roller 27 isregulated in thickness by a blade 32.

The electrostatic latent image formed on the surface of thephotosensitive drum 23 is visualized as a toner image, as a result ofthe developer roller 27 attaching toner to the electrostatic latentimage. The toner image, upon formed on the photosensitive drum 23 as aresult of the toner attachment, is transferred onto the recording sheetP during a passing of the recording sheet P between the photosensitivedrum 23 and the transfer roller 25 to which a transfer bias opposite inpotential to that of the photosensitive drum 23 has been applied. Upontransfer, a toner remaining on the surface of the photosensitive drum 23is temporarily collected by the cleaning roller 30 to return to thephotosensitive drum 23 at a predetermined time, and is then collectedinto the process unit 3 by the developer roller 27.

A toner sensor 33 is disposed at the upper support plate 2 a of thescanner unit 2 so as to project upwardly therefrom. The toner sensor 33is configured to include a light emitter and a light receiver, althoughare not shown, which coact to detect optically whether or not a toner isresident in the toner cartridge 29. The toner sensor 33 is disposed toface a recess, as not shown, formed on the lower surface portion of thetoner cartridge 29 in the process unit 3.

The process unit 3 is in the form of a cartridge allowing the componentsof the process unit 3 to be contained within the case 34 made ofsynthetic resin. The thus-cartridged process unit 3 is removably mountedon the main frame 1 a.

As shown in FIG. 2, in the laser printer, a storage portion 36 is formedin an area in which a front portion of the main frame 1 a (theright-hand portion of the main frame 1 a in FIG. 2) and a front portionof the main cover 1 b (the right-hand portion of the main cover 1 b inFIG. 2) are connected with each other. The storage portion 36 is locatedunder a bottom portion of the main frame 1 a. Within the storage portion36, a cooling fan 35 is accommodated.

As shown in FIG. 2, an air duct 37 communicating with a space within thestorage portion 36 is formed so as to extend in the lateral direction ofthe laser printer. The lateral direction is parallel to the widthwise ofa recording sheet P traveled along the aforementioned travel path, andperpendicular to the sheet of FIG. 2. As shown in FIGS. 1-2, the airduct 37 includes a member 37 a extending in the lateral direction of thelaser printer with its inverted-V shaped section. The member 37 a ispositioned between the process unit 3 and the fuser unit 4 so as tothermally isolate the heat roller 15 from the process unit 3, with theresult that heat generated by the heat roller 15 of the fuser unit 4 isprevented from being conducted or transmitted directly to the processunit 3.

As shown in FIG. 2, a position of a cooling air flow generated by thecooling fan 35 flows along a lower surface of a bottom plate portion 38of the main frame 1 a into the rear portion of the laser printer,thereby cooling a power source 39 and the main motor 6 a within thedrive train unit 6 both of which are disposed at the rear portion of thelaser printer.

As shown in FIG. 2, another portion of the cooling air flow generated bythe cooling fan 35 flows via the air duct 37 from the right-hand frontportion of the laser printer at which the cooling fan 35 is disposed, tothe left-hand front portion of the laser printer. The another portion ofthe cooling air flow subsequently flows into the rear portion of thelaser printer via an inner space extending from the right-hand frontportion to the right-hand rear portion of the laser printer. The coolingair flow functions to reduce rise in temperature of the member 37 a overthe entire length thereof.

As shown in FIG. 2, still another portion of the cooling air flowgenerated by the cooling fan 35 blows out through a plurality of slits39 a of the member 37 a open to the process unit 3. The still anotherportion of the cooling air flow rises up so as to pass through betweenthe process unit 3 and the fuser unit 4. Eventually, the still anotherportion of the cooling air flow exits from the laser printer via aplurality of air outlets 40 (see FIG. 1) formed in the top cover 7.

The constructions equivalent or similar to those for avoiding rise intemperature of the laser printer are disclosed in U.S. Pat. No.6,205,302, the content of which is incorporated hereinto by reference

FIG. 4 shows the motor control device of the laser printer. In thepresent embodiment, although the motor control device is employed forthe drive control of at least one of the main motor 6 a and the scannermotor 86, the motor control device may of course be employed for thedrive control of other motors of the laser printer.

The main motor 6 a and the scanner motor 86 are each in the form of athree-phase brushless DC motor. The main motor 6 a and the scanner motor86 are also common to each other in that they are motors controlled bythe motor control device. In view of this, they will be collectivelyreferred to as “motor 100.”

The motor control device is configured so as to include: a frequencygenerator (hereinafter, referred to simply as “FG”) 150: an amplifier190; a speed controller 200; a low-pass filter (hereinafter, referred tosimply as “LPF”) 290; and a motor driver 300.

The FG 150, provided for the motor 100, outputs as a frequency signal ananalog signal varied in frequency depending on the rotational speed ofthe motor 100. The frequency signal outputted from the FG 150 isamplified by the amplifier 190. The amplifier 190 amplifies thefrequency signal received from the FG 150 by an extremely largeamplification factor, to thereby obtain a signal approximate to a binarysignal to be obtained by a literal thresholding operation of the rawfrequency signal.

The speed controller 200 in the form of an Application-SpecificIntegrated Circuit (ASIC) includes: an input buffer 205; a noisereducing section 210; a set value holder 220; and a set value modifier230.

The input buffer 205, once receiving the output signal of the amplifier190, amplifies the output signal by an extremely large amplificationfactor. As a result of the amplification, the output signal of theamplifier 190, as described later with reference to FIG. 5, is processedto become a binary signal.

The input buffer 205 is provided as a CMOS buffer, for example. Inoperation, the input buffer 205 outputs a signal at low level if theinput signal of the input buffer 205 is not higher than an lowerthreshold value, while outputs a signal at high level if the inputsignal of the input buffer 205 is not lower than an upper thresholdvalue.

For this reason, where the input signal of the input buffer 205 is not aliteral binary signal and therefore takes any level between the abovetwo threshold values, which is to say, where the wave form representedby the input signal of the input buffer 205 has inclined portionsthereof, as shown in FIG. 5(b), for example, the corresponding outputsignal of the input buffer 205 fails to be stable.

In this case, the output signal of the input buffer 205 is varied inlevel depending on various statuses of the voltage of the related powersource, the input buffer 205, etc., resulting in local oscillations ofthe output signal of the input buffer 205, as shown in FIG. 5(c), forexample.

The local oscillations result from the above inclined portions of thewave form representative of the input signal of the input buffer 205,i.e., the output signal of the amplifier 190. The lower the frequency ofthe output signal of the FG 150, the longer in time the inclinedportions each continue. The local oscillations of the output signal ofthe input buffer 205 correspond to a noise component which has beenincorporated into the frequency signal. The frequency signal is one thatoriginated from the FG 150, and that will enter the noise reducingsection 210.

The noise reducing section 210 is provided to reduce the level of anoise component which has been incorporated into the frequency signalreceived from the input buffer 205. The noise component which is to bereduced in the frequency signal by the noise reducing section 210 isvaried in characteristics (e.g., frequency) depending on a set valueentering the noise reducing section 210.

The set value modifier 230, as described later in greater detail, isprovided to modify the set value. On the other hand, the set valueholder 220 is provided to hold the set value set by the set valuemodifier 230 and to deliver the set value to the noise reducing section210.

The speed controller 200 further includes a period measuring section240. The period measuring section 240 measures the period of thefrequency signal (corresponding to the rotational speed of the motor100) from the frequency signal in which the noise component has beenreduced by the noise reducing section 210.

The speed controller 200 yet further includes a desired value settingsection 250, a comparator 260, a speed computing section 270, and apulse width modulator 280, all of which are for the feedback control ofthe rotational speed of the motor 100. Their constructions, as arewell-known in the art, will be described below briefly.

The desired value setting section 250 establishes a desired value forthe feedback control of the rotational speed of the motor 100. Thecomparator 260 compares the established desired value and the periodmeasured by the period measuring section 240 with each other, and thenoutputs a signal representative of the result of the comparison to thespeed computing section 270.

Based on the output of the comparator 260 (in the form of digital data),the speed computing section 270 performs a known computing processrequired for controlling the motor 100 through feedback control such asa PID control (Proportional Integral Derivative Control), resulting inoutputting of control data indicative of the instructions to the motor100. The pulse width modulator 280 modulates the control data receivedfrom the speed computing section. 270, in a PWM manner. The LPF 290,once receiving a control signal (in the form of an analog signal) fromthe pulse width modulator 280, generates an analog voltage signal whichis to be actually delivered to the motor driver 300.

The motor driver 300 retrieves the analog voltage signal generated bythe LPF 290. The motor driver 300 includes a PWM (pulse width modulated)signal generator 310; a conduction logic circuit 320; an amplifier 330;and a driver 340. Their constructions, as are well-known in the art,will be described below briefly.

The PWM signal generator 310 generates the PWM signal for use in actualcontrol of the rotational motion of the motor 100, by superimposing theretrieved analog voltage signal on a carrier wave signal having apredetermined carrier frequency. The amplifier 330 amplifies the outputsignals of hall elements 110 (HU), 120 (HV), 130 (HW), all of which aremounted on the motor 100.

The conduction logic circuit 320 determines the amounts of currents tobe supplied to coils U, V, W of the motor 100, respectively, based onboth the FWM signal retrieved from the PWM signal generator 310 and thethree output signals delivered from the amplifier 330 for the respectivehall elements 110, 120, 130. The conduction logic circuit 320 thenoutputs to the driver 340 drive signals indicative of the determinedamounts of currents. Based on the drive signal retrieved from theconduction logic circuit 320, the driver 340 supplies drive currents tothe coils U, W, W, respectively, whereby the motor 100 is driven.

The frequency signal generated by the FG 150 is a signal smaller inamplitude and lower in frequency during a start up of the motor 100. Asthe motor 100 is shifting to a regular operation state (i.e., a stableoperation state) thereof, the frequency signal increases in amplitude,and increases in frequency depending on the rotational speed of themotor 100.

During a start up of the motor 100, a case may exist where therotational motion of the motor 100 incorporates therein the noisecomponent resulting from unintended reverse rotations of the motor 100or the like due to for example backlash of the gear train 6 b or thelike. For this reason, during a start up of the motor 100, thepossibility arises that the frequency signal does not always reflectaccurately a true one of the components of the compound rotationalmotion of the motor 100.

With this in mind, in the present embodiment, the noise component isreduced in the frequency signal by means of the noise reducing section210, allowing the control of the rotational motion of the motor 100 withthe rotational state of the motor 100 being accurately monitored.

FIG. 5 exemplifies in wave form charts the output signals of the FG 150,the amplifier 190, the input buffer 205, and the noise reducing section210, for the sake of the explanation of a noise reduction functionprovided by the noise reducing section 210.

The wave form of FIG. 5(a), as is a representation of an example of theraw frequency signal, shows that the increase in rotational speed of themotor 100 with time occurs with the increase in frequency of thefrequency signal. For the sake of simplicity, FIG. 5(a) is omitted toillustrate changes in amplitude of the frequency signal according to therotational speed of the motor 100.

The wave form of FIG. 5(b), as is a representation of the output signalof the amplifier 190, shows that, in the present embodiment, asdescribed above, the extremely large amplification factor of theamplifier 190 results in the output signal of the amplifier 190 beingapproximate to a typical binary signal, while the length of time tduring a portion (i.e., the aforementioned inclined portion) of theoutput signal of the amplifier 190 which changes from high to low levelvaries depending on the frequency of the frequency signal.

In the present embodiment, as described above, the output signal of theamplifier 190 is converted into the corresponding binary signal by meansof the input buffer 205 in preparation for the delivery to the speedcontroller 200. In the case where the frequency signal entering theamplifier 190 incorporates therein a noise component, as shown in FIG.5(c), the conversion, i.e., a thresholding operation may cause theoutput signal of the input buffer, that is, the input signal of thenoise reducing section 210 to contain a noise. The noise reducingsection 210 performs a noise reduction processing for the input signalof the noise reducing section 210, to thereby generate the signalsimilar to that of the wave form shown in FIG. 5(d), and to transmit thesignal to the period measuring section 240.

Then, the noise reduction processing implemented by the noise reducingsection 210 will be described in greater detail.

The noise reduction processing in the present embodiment is implementedto reduce the noise shown in FIG. 5(c), that is, the phenomenon in whichthe input signal of the noise reducing section 210 oscillates betweenhigh and low level.

For implementing the noise reduction processing, the noise reducingsection 210 includes an up/down counter in the form of a hard-wiredlogic circuit. The counter is clocked by a reference clock signal of thesystem, and selectively performs an incremental and a decrementalcounting, depending on whether the input signal of the noise reducingsection 210 is at high or low level Specifically, the counter's countvalue ranges from “0” to the same value as the set value held by the setvalue holder 220. If the input signal of the noise reducing section 210is at high level, the counter is incremented by a logic one each cycleof the reference clock signal of the system, with its maximum valuebeing equal to the same value as the set value held by the set valueholder 220. On the contrary, if the input signal of the noise reducingsection 210 is at low level, the counter is decremented by a logic oneeach cycle of the reference clock signal of the system, with its minimumvalue being equal to “0.”

Once the incremental operation of the counter causes the count value toreach the set value held by the set value holder 220, the output signalof the noise reducing section 210 becomes high in level. On the otherhand, once the decremental operation of the counter causes the countvalue to reach “0,” the output signal of the noise reducing section 210becomes low in level.

Such a processing allows the noise reducing section 210 to function as alow pass filter, to thereby avoid the noise existing in the input signalof the noise reducing section 210 during a high to low change ortransition of the input signal, from entering, without being subjectedto reduction, the subsequent-stage circuitry including the periodmeasuring section 240.

The relationship between the input and output signals of the noisereducing section 210, that is, the characteristics of the noisecomponent to be reduced by the noise reducing section 210, variesdepending on the set value held by the set value holder 220.

FIG. 6 exemplifies in wave form charts three signals for explaining howthe characteristics of the noise to be reduced by the noise reducingsection 210 change according to the magnitude of the set value. FIG.6(a) shows an example of the wave form indicative of the input signal ofthe noise reducing section 210, FIG. 6(b) shows an example of the waveform indicative of the output signal of the noise reducing section 210where the set value is smaller, and FIG. 6(c) shows an example of thewave form of the output signal of the noise reducing section 210 wherethe set value is larger.

As shown in FIG. 6(a), the noise contained in the input signal of thenoise reducing section 210 at the time of its high to low change ortransition in level, irrespective of whether the set value is smaller orlarger, does not immediately appear in the output signal of the noisereducing section 210, as shown in FIGS. 6(b) and 6(c).

However, where the set value is set to be smaller, the output signal ofthe noise reducing section 210 makes a high to low transition only alittle after the input signal of the noise reducing section 210 makes ahigh to low transition, as shown in FIG. 6(b).

It is added that, a schumitt trigger may be applied as an alternativemanner for reducing of such a noise. However, noises due to backlash,etc. may include a kind of noise that the hysteresis of the schumitttrigger fails to cover. It is also added that, in the presentembodiment, the adjustment of the set value of the noise reducingsection 210 allows a relatively flexible modification of the kind of areducible noise component. Therefore, although the present invention maybe practiced using the schumitt trigger as a noise reduction, the noisereduction processing employed in the present embodiment is also useful.

On the other hand, where the set value is set to be larger, the periodof one cycle of the output signal of the noise reducing section 210becomes longer than that of the input signal as shown in FIG. 6(c). Forthis reason, a high to low transition of the output signal is made atthe point B of time much later than the point A of time at which a highto low transition of the input signal is made, as compared with that ofFIG. 6(b). During a period between these points A and B, the noiseincorporated into the input signal is reduced, and however, a true oneof the frequency components of the composite input signal, in additionto the noise, may be reduced.

What value the set value holder 220 is to hold as the set value, andwhat value the set value modifier 230 is to modify the initial set valueinto after a predetermined length of time elapses since the start up ofthe motor 100 each depend on the frequency of the binary signal intowhich the frequency signal has been converted, the frequency of a trueone of the frequency components of the composite frequency signal, etc.

It is preferable that the set value is set such that a targetedcomponent of the input signal of the noise reducing section 210 that isrequired for detecting the true one of the components of the compositerotational motion of the motor 100 is not reduced but passed through thenoise reducing section 210, while a high-frequency noise component ofthe same input signal is reduced, especially during the start up of themotor 100, and such that the rotational state represented by theresulting output signal of the noise reducing section 210 is notexcessively different from the corresponding actual rotational state ofthe motor 100.

The set value held by the set value holder 220 may be set such that theset value remains unchanged, irrespective of whether it is during orafter the start up of the motor 100, during the operation of the laserprinter. Alternatively, the set value may be set so as to vary with timedepending on whether it is during or after the start up of the motor100.

The modification of the set value is made by the set value modifier 230.In this case, the set value modifier 230 may be operated by a computer.The set value modifier 230 may be configured, for example, so as to setthe set value to a larger value during the start up of the motor 100,and to modify the initial set value into a smaller value after apredetermined length of time elapses from the start up of the motor 100.

The signal in which the noise component has been reduced by the noisereducing section 210 enters in the form of a digital signal (an exampleof the signal of the rotational state) the period measuring section 240.The period measuring section 240 measures the period of each on/offcycle of the digital signal entered, and outputs to the comparator 260the measured period in the form of digital data having a predeterminedbit number (8 or 16 bits, for example).

The comparator 260 compares a value represented by the digital data(which reflects the rotational speed of the motor 100) received from theperiod measuring section 240 with the desired value set by the desiredvalue setting section 250, resulting in the delivery of the dataindicative of the result of the comparison to the speed computingsection 270.

The speed computing section 270 refers to the result of the comparisonbetween the rotational speed of the motor 100 detected by the periodmeasuring section 240 and the desired value, to thereby generate controldata indicative of the instructions to the motor 100 required for thecontrol of the motor 100 using a known control manner such as a PIDcontrol, for example.

The bit number of the control data may be determined as desiredaccording to the frequency of the reference clock of the system, forexample. The control data may therefore be generated in the form ofdigital data 16 bits, 32 bits, or 64 bits long, for example. Thegenerated control data, upon delivered to the pulse width modulator 280,is modulated into a pulse width modulated signal (PWM signal) by thepulse width modulator 280.

As is evident from the above explanation, the motor control device inaccordance with the present embodiment, by virtue of the noise reducingsection 210 reducing the noise component appearing in the binary signaldelivered to the speed controller 200, permits the feedback control ofthe motor 100 with the use of the frequency signal representative of therotational state of the motor 100, even during the start up of the motor100, with the rotational state of the motor 100 being accuratelymonitored.

In other words, the above motor control device permits the feedbackcontrol of the motor 100 with the use of the frequency signalrepresentative of the rotational state of the motor 100, irrespective ofwhether or not the start up of the motor is being experienced.

As will be readily understood from the above explanation, in the presentembodiment, the FG 150 constitutes an example of the “signal generator”set forth in the above mode (1), and the amplifier 190, the input buffer205, the speed controller 200, the LPF 290, and the motor driver 300cooperate with each other to constitute an example of the “controller”set forth in the same mode.

Further, in the present embodiment, the noise reducing section 210, theset value holder 220, and the set value modifier 230 cooperate with eachother to constitute an example of the “noise reducer” set forth in theabove mode (3), the period measuring section 240, the desired valuesetting section 250, the comparator 260, the speed computing section270, and the pulse width modulator 280 cooperate with each other toconstitute an example of the “control-signal producing device” set forthin the same mode.

Yet further, in the present embodiment, the scanner unit 2, the processunit 3, and the fuser unit 4 cooperate with each other to constitute anexample of the “image forming device” set forth in the above mode (16),the feeder unit 5 constitutes an example of the “feeding device” setforth in the same mode, the main motor 6 a and the scanner motor 86cooperate with each other to constitute an example of the “at least onemotor” set forth in the same mode, and the FG 150, the amplifier 190,the input buffer 205, the speed controller 200, the LPF 290, and themotor driver 300 cooperate with each other to constitute an example ofthe “driving device” set forth in the same mode.

Then, with reference to FIGS. 7-9, a second embodiment of the presentinvention will be described. However, in the present embodiment, thereare many elements common to the first embodiment. In view of the above,the common elements of the present embodiment to those of the firstembodiment will be referenced the same names or the same referencenumerals as those in the description and illustration of the firstembodiment, without a redundant description and illustration, while thedifferent elements of the present embodiment from those of the firstembodiment will be described in more detail.

In the first embodiment, the rotational state of the motor 100 isdetected with the use of the frequency signal outputted from the FG 150provided for the motor 100. On the other hand, in the presentembodiment, the rotational state of the scanner motor 86 rotating thepolygon mirror 20 is detected with the use of a signal outputted from abeam detector detecting the laser beam emitted from the polygon mirror20.

FIG. 7 shows in perspective view an exposure section of the laserprinter including a motor control device in accordance with the presentembodiment. In the exposure section, a light source 80 including asemiconductor laser, etc. emits the laser beam (denoted by “LB” in FIG.7) which is deflected by the polygon mirror 20 driven for rotation bythe scanner motor 86. The deflected laser beam passes through an f θlens 21 into the surface of the photosensitive drum 23, whereby thephotosensitive drum 23 is cyclically scanned with and exposed to thelaser beam in the primary scanning direction.

As shown in FIG. 7, in the laser printer, a beam detector (hereinafter,referred to simply as “BD”) 81 is disposed stationary at a predeterminedposition for allowing the laser beam to enter the BD 81, each cycle ofscanning of the laser beam immediately prior to the start point of acorresponding cycle of scanning of the photosensitive drum 23. The BD 81outputs a detector signal varied in level depending on whether or notthe BD 81 receives the laser beam. The detector signal, as shown in FIG.9(a), becomes an off-state signal (a non-light-reception signal) in astate in which the BD 81 does not receive the laser beam, while thedetector signal becomes an on-state signal (a light-reception signal) ina state in which the BD 81 receives the light beam.

FIG. 8 shows in block diagram an example of a motor control device ofthe present embodiment in a similar manner to FIG. 4. For explaining anoise reduction processing of the motor control device, FIG. 9 showsseveral signals in wave form charts. FIG. 9(a) shows a wave form of theBD signal, which is outputted from the BD 81 and which becomes anon-state signal at the point of time at which the laser beam enters theBD 81.

Because of the fast passing of the laser beam through the BD 81 duringthe scanning, the BD signal, even after becoming on-state accordingly,remains the same in state only for very short period, as shown in FIG.9(a).

For this reason, if the noise reducing section 210 in the presentembodiment implements the noise reduction processing in the same manneras that in the first embodiment without any additional processing, itmay possibly cause the output signal of the noise reducing section 210to show no temporal change and eventually no reflection of actualchanges in the BD signal.

In view of the above, in the present embodiment, the BD signal isprocessed, such that, once the state of the unprocessed or raw BD signalis changed from an off-state to an on-state, the state of the processedBD signal, upon changed from an off-state to an on-state, is retained tobe an on-state even after the unprocessed BD signal is returned to anoff-state, to thereby prolong a period during which the state of theprocessed BD signal is an on-state. This pre-processing secures thesubsequent noise reduction processing for the BD signal.

As shown in FIG. 8, the speed controller 200 in the present embodimentincludes a BD signal holder 410 and a counter 420, in addition to thesame component as those of the speed controller 200 shown in FIG. 4.

In the BD signal holder 410, if the BD signal changes from an off-stateto an on-state at the point A of time as shown in FIG. 9(a), the outputsignal of the BD signal holder 410 changes from an off-state to anon-state as shown in FIG. 9(b) as well. In the BD signal holder 410,even after the BD signal returns to an off-state, the output signal ofthe BD signal holder 410 is retained to be an on-state for a period aslong as possible. As a result, a period during which a signal isretained to be an on-state (hereinafter, referred to as “on-stateperiod”) becomes longer with the output signal of the BD signal holder410 than with the raw BD signal.

More specifically, the BD signal holder 410 is operated, for prolongingthe on-state period of its output signal, such that, for example, oncethe BD signal becomes an on-state, the BD signal is latched, to therebyhold the output signal of the BD signal holder 410 to be an on-stateuntil the counter 420 times-up, i.e., until the point B of time isreached as shown in FIG. 9.

As shown in FIG. 8, the output signal of the ED signal holder 410 thenenters the noise reducing section 210 implementing the noise reductionprocessing in the same manner as that of the first embodiment. As aresult, as shown in FIG. 9(c), the reduction of a noise component of theoutput signal of the noise reducing section 210 is performed in thevicinity of the point A of time.

As is evident from the above explanation, in the present embodiment, theBD 81 constitutes an example of the “signal generator” set forth in theabove mode (1), and the amplifier 190, the input buffer 205, the speedcontroller 200, the LPF 290, and the motor driver 300 cooperate witheach other to constitute an example of the “controller” set forth in thesame mode.

Further, in the present embodiment, the noise reducing section 210, theset value holder 220, the set value modifier 230, the BD signal holder410, and the counter 420 cooperate with each other to constitute anexample of the “noise reducer” set forth in the above mode (3), theperiod measuring section 240, the desired value setting section 250, thecomparator 260, the speed computing section 270, and the pulse widthmodulator 280 cooperate with each other to constitute an example of the“control-signal producing device” set forth in the same mode.

Yet further, in the present embodiment, the BD signal holder 410 and thecounter 420 cooperate with each other to constitute an example of the“signal processor” set forth in the above mode (15), the noise reducingsection 210, the set value holder 220, and the set value modifier 230cooperate with each other to constitute an example of the “noisereducing device” set forth in the same mode, and the period measuringsection 240, the desired value setting section 250, the comparator 260,the speed computing section 270, and the pulse width modulator 280cooperate with each other to constitute an example of the“control-signal producing device” set forth in the same mode.

It is added that, although the noise reducing section 210 introduces atime delay of a transition point of time at which the output signal ofthe noise reducing section 210 changes between an off-state and anon-state, relative to a transition point of time at which the BD signalchanges between an off-state and an on-state, the period T of the outputsignal of the noise reducing section 210 approximately coincides withthat of the BD signal.

In addition, for securing the speed control of the scanner motor 86,accuracy in measuring the period T of the output signal of the noisereducing section 210 is important to be achieved. For this reason, theabove time delay of the transition point of the output signal of thenoise reducing section 210 can be ignored without causing any practicalproblem.

Then, a third embodiment of the present invention will be described.However, in the present embodiment, there are many elements common tothose of the first and second embodiments. In view of this, the commonelements of the present embodiment will be referenced the same names orthe same reference numerals as those in the description and illustrationof the first and second embodiments, without a redundant description andillustration, while the different elements of the present embodimentfrom those of the first and second embodiments will be described in moredetail.

The first and second embodiments are each constructed, as describedabove, such that the noise reducing section 210 delays the transitionpoint of the output signal of the noise reducing section 210 until thesum of lengths in time of durations, during each of which the inputsignal of the noise reducing section 210 is held at a selected one of ahigh level and a low level reaches a length of time corresponding to theabove-described set value, to thereby implement the noise reductionprocessing for the frequency signal of the FG 150 and the BD signal ofthe BD 81.

On the other hand, where the frequency signal of the FG 150 is employedfor controlling the motor 100, there is a characteristic that thefrequency signal is smaller in amplitude during a start up of the motor100 and that the frequency signal increases in amplitude as the motor100 is shifting from a start-up or transient state to a regular orsteady rotational state.

By the use of the above characteristic, the noise reduction can beprovided by temporally changing the characteristic of the amplitude 190without requiring such a processing as the noise reducing section 210implements.

With this in mind, in the present embodiment, the amplifier 190 is inthe form of an amplifier (antilogarithmic amplifier, for example) whichamplifies an input signal thereof by a variable amplification factor,wherein the amplification factor is established to become smaller duringa period with the input signal smaller in amplitude, and become largerduring a period with the input signal larger in amplitude. The amplifier190 provides the noise reduction during a start up of the motor 100.

In the present embodiment, the noise reducing section 210, while may beremoved from the speed controller 200, may of course be includedtherein.

As is evident from the above explanation, in the present embodiment, theamplifier 190 constitutes an example of the “amplifier” set forth in theabove mode (8) or (9).

Then, a forth embodiment of the present invention will be describedbelow. However, in the present embodiment, there are many elementscommon to those of the third embodiment. In view of the above, thecommon elements of the present embodiment will be referenced the samenames or the same reference numerals as those in the description andillustration of the third embodiment, without a redundant descriptionand illustration, while the different elements of the present embodimentfrom those of the third embodiment will be described in more detail.

In the present embodiment, the amplifier 190 is in the form of anamplifier variable in state between an operative state and aninoperative state. The amplifier 190 is set to be placed in aninoperative state during a start up of the motor 100 for preventingamplification of an input signal of the amplifier 190. Owing to thissetting, the amplifier 190 provides the noise reduction during a startup of the motor 100.

In the present embodiment, the noise reducing section 210, while may beremoved from the speed controller 200, may of course be includedtherein.

As is evident from the above explanation, in the present embodiment, theamplifier 190 constitutes an example of the “amplifier” set forth in theabove mode (7).

Although the several embodiments of the present invention have beendescribed above, the present invention is of course not limited to thespecific details and representative embodiments shown and describedherein. Accordingly, for example, various modifications may be madewithout departing from the spirit or scope of the present invention.

For example, in the first to fourth embodiments, the noise reducingsection 210 is constructed, such that the counter thereof performs acounting operation, in response to each change in level of the inputsignal of the noise reducing section 210, in a corresponding one ofcounting directions (for incremental (up) and decremental (down)counting operations, respectively) to an actual one of transitiondirections of the input signal (high-to-low direction and low-to-highdirection).

The noise reducing section 210 is further constituted, such that theoutput signal of the noise reducing section 210 is fixed in level untilthe count value of the counter reaches the set value, and such that theoutput signal is allowed to change in level after the count value of thecounter exceeds the set value.

Additionally or alternatively to the noise reducing section 210, adigital filter such as a well-known FIR (finite-duration impulseresponse)-type or IIR (infinite-duration impulse response)-type may beemployed so as to temporally change the filter factor of the digitalfilter for achieving noise reduction similar to that of the noisereducing section 210, to practice the present invention.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. An apparatus for controlling a rotational motion of a motor,comprising: a signal generator generating a rotational state signalindicative of a rotational state of the motor: and a controllerproducing a control signal for controlling the rotational motion of themotor, based on the rotational state signal generated by the signalgenerator, to thereby control the motor based on the produced controlsignal, wherein the controller produces the control signal during astart up of the motor, such that the produced control signal is notaffected by a noise component which is incorporated into the rotationalstate signal during the start up of the motor.
 2. The apparatusaccording to claim 1, wherein the controller produces the control signalfor controlling the rotational motion of the motor through feedbackcontrol based on the generated rotational state signal, irrespective ofwhether or not the start up of the motor is being experienced.
 3. Theapparatus according to claim 1, wherein the controller comprises: anoise reducer reducing a level of the noise component in the rotationalstate signal generated by the signal generator; and a control-signalproducing device producing the control signal based on the rotationalstate signal in which the noise component has been reduced by the noisereducer.
 4. The apparatus according to claim 3, wherein the noisereducer reduces the level of the noise component in the rotational statesignal, prior to the production of the control signal based on therotational state signal.
 5. The apparatus according to claim 3, whereinthe noise reducer is configured such that a noise component to bereduced in level in the rotational state signal is varied incharacteristics depending on a set value, and wherein the set value isvaried to be different between during and after the start up of themotor.
 6. The apparatus according to claim 5, wherein the set value isestablished in magnitude to allow the noise reducer not to reduce thelevel of a frequency component of the rotational state signal whichreflects a true component of compound rotational motion of the motorduring the start up of the motor.
 7. The apparatus according to claim 3,wherein the noise reducer comprises an amplifier amplifying therotational state signal, wherein the amplifier is configured to becomesubstantially inoperative during the start up of the motor.
 8. Theapparatus according to claim 3, wherein the noise reducer comprises anamplifier amplifying the rotational state signal by a variableamplification factor, wherein the amplification factor is established tobecome smaller during the start up of the motor, and become largerduring a steady motion of the motor.
 9. The apparatus according to claim8, wherein the amplifier is an antilogarithmic amplifier.
 10. Theapparatus according to claim 1, wherein the signal generator comprises afrequency generator outputs the rotational state signal in the form of afrequency signal which is an analog signal varied in frequency dependingupon the rotational state of the motor.
 11. The apparatus according toclaim 10, wherein the frequency signal is varied in frequency dependingupon a rotational speed of the motor.
 12. The apparatus according toclaim 10, wherein the controller comprises a noise reducing devicereducing in level the noise component in a signal obtained by amplifyingthe frequency signal generated by the frequency generator.
 13. Theapparatus according to claim 1, wherein the motor comprises a scannermotor rotating a polygon mirror for use in scanning a light beancyclically, wherein the signal generator comprises a beam detectordisposed stationary at a predetermined position allowing the light beamenters the beam detector intermittently per each cycle of scanning ofthe light beam, the beam detector outputting a detector signal varied inlevel depending on whether or not the beam detector receives the lightbeam, and wherein the rotational state signal is the detector signaloutputted from the beam detector.
 14. The apparatus according to claim13, wherein the polygon mirror deflects the light beam cyclically toscan a photoconductor.
 15. The apparatus according to claim 13, whereinthe controller comprises: a signal processor processing the detectorsignal outputted from the beam detector: a noise reducing devicereducing the noise component in the detector signal processed by thesignal processor; and a control-signal producing device producing thecontrol signal based on the detector signal in which the noise componenthas been reduced by the noise reducing device, wherein the signalprocessor processes unprocessed detector signal into processed detectorsignal in a manner that, in a steady state of the unprocessed detectorsignal, the processed detector signal indicates whether the unprocesseddetector signal is a non-light-reception signal indicating that the beamdetector does not receive the light beam, or a light-reception signalindicating that the beam detector receives the light beam, and whereinthe signal processor processes the unprocessed detector signal into theprocessed detector signal in a manner that, in an oscillating state inwhich the unprocessed detector signal is oscillated in level so as toalternately repeat a forward transition of the unprocessed detectorsignal from the non-light-reception signal to the light-receptionsignal, and a reverse transition from the light-reception signal to thenon-light-reception signal, the processed detector signal steadilyindicates the forward transition, without indicating the reversetransition, during a predetermined length of time elapsed from a starttime of the forward transition.
 16. An apparatus for forming an image,comprising: an image forming device forming an image on an imagereceiver medium; a feeding device feeding the image receiver medium tothe image forming device; at least one motor for use in at least one ofthe image forming device and the feeding device; and a driving devicedriving the at least one motor for rotation thereof, wherein the drivingdevice comprises the apparatus according to claim 1 for use incontrolling at least one of the at least one motor.
 17. A method ofcontrolling a rotational motion of a motor, comprising the steps of:generating a rotational state signal indicative of a rotational state ofthe motor; reducing within the generated rotational state signal a levelof a noise component which is incorporated into the rotational statesignal during a start up of the motor; producing a control signal forcontrolling the rotational motion of the motor through feedback control,based on the rotational state signal in which the noise component hasbeen reduced in level, irrespective of whether or not the start up ofthe motor is being experienced; and controlling the motor based on theproduced control signal.